This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Ghost imaging is the term that has become popular for imaging with photon correlations instead of accumulating single-photon events. There are two types of ghost imaging-type 1 is based on entangled photons, and type 2 on intensity correlations in thermal light of the type first used in the famous Hanbury Brown-Twiss (HBT) experiment. The thermal-light ghost imaging (TGI) is the focus of this application. In this context, the term “thermal” does not refer to infrared wavelengths, but rather the photon statistics of the light emitted from a glowing-hot surface as opposed to, for example, laser light. TGI is capable of forming an image where conventional imaging based on single photons is not, due to fluctuating distortions in the optics, for example, from a turbulent atmosphere. As will be described below, TGI relies on a combination of narrow spectral bandwidth and a high temporal resolution of imaging detectors. The faster the detector, the more spectral bandwidth is admissible. TGI offers some advantages over conventional imaging, such as being minimally affected by turbulent atmospheric conditions, but requiring more sophisticated instrumentation.
The original Hanbury Brown-Twiss experiment measured the angular diameter of a star (Sirius A), which, at about 6 milli-arcseconds, was below the resolution limit of telescopes in the 1950s. It did so by determining intensity correlations from the same source seen from two telescopes as a function of their spatial separation. The correlation drop-off over the separation is reciprocal to the angular range of light coming from different parts of the stellar disk. The intensities are correlated only over times that are reciprocal to the spectral bandwidth of the light, as set by filters on the telescopes. A narrow spectral bandwidth gives rise to longer coherence times (lengths of the wavepackets), and correspondingly slower detectors can detect correlations. Even if the temporal resolution of the detectors does not match the reciprocal spectral bandwidth, correlations are present, but at a reduced contrast, such as in the original HBT experiment. The remarkable fact about this experiment is that the signal contrast is not affected by atmospheric “seeing”, i.e., the image distortions from variable refractive-index gradients in the turbulent atmosphere. These affect both photons in the same way, and thus cancel out. If the detectors have a temporal resolution that is insufficient, then the interference contrast will be reduced as there are many chance coincidences on top of the meaningful ones. One can then reduce the bandwidth of the light by choosing narrower filters, use a light source that emits very narrowband light (not an option in astronomy), or use a faster detector.
Type-2 ghost imaging is conceptually rather similar. In a typical configuration (see FIG. 4), a fast “bucket detector” with no spatial resolution is used to gate acquisition with an imaging detector. Each time the bucket detector registers a photon, the location of the corresponding photon on the imaging detector is determined. This splitting of functions was necessary because no fast imaging detector was available. Furthermore, all demonstration experiments of thermal-light ghost imaging used some kind of very narrow-band light source, such as a laser with means to make the spatial mode appear like thermal light, a so-called pseudo-thermal light source, or a hollow-cathode lamp. The only demonstration of GI with spectrally filtered sunlight was done with a pair of point detectors, i.e., detectors lacking spatial resolution.
A need exists for improved technology, including technology that can perform imaging at temporal resolutions approaching the reciprocal bandwidth of commercially available interference filters.